I cant say I've had any English IPA, just goes to show you how America has really adopted the style that once belonged the the British. I would guess the difference would be in the hops, with an American IPA being a more in-your-face hop bonanza, and the English being a bit more malty and smooth.
the base grain is different for starters. the british pale ale malt will give more a of a "biscuity" flavor than domestic 2-row. and definitely the hops are different. not only in IBU and dry hopping but in variety as well. you'll see more EKG and other british hops used in their IPAs and more of the grapefruity, piney hop dominance in american IPAs. pretty much every aspect of american IPAs are bigger than the british counterparts. yeast can be a difference too, depending on who makes it. a lot of american breweries use english ale yeasts for their IPAs and they're still considered american, but i guess on average, american ones will use a cleaner yeast (like wyeast 1056 vs. 1968) but again, that all depends on the brewery.
As far as I am concerned, English and American IPAs are very different beers. I love them both, but I consider an English IPA a slightly higher ABV and hoppier beer than an ESB (using English malt and hops); while an American IPA is a highly hopped, higher ABV, American Pale Ale (with American hops). [IIPA is of course the same as an AIPA, but more so.]
If I ruled the BJCP, English IPA would be with the English Pale Ales (which would be called by their proper name: bitters (or simply "beer")) and American IPA would be in an American Pale Ale category along with Imperial IPAs. Not sure where American Ambers and Browns would go though...
Paraphrased from Brewing Classic Styles (Palmer,Zainasheff 2007)
There are 3 IPA's. In increasing hoppiness: English, American, and Imperial.
The English IPA uses English grains, Pale Ale Malt and as opposed to other English beers the late hop character is more pronounced. English needs a dry finish so pick an attenuative yeast. Lower mash temps are recommended to help achieve the lower FG.
60 min boil, preboil volume 7G, preboil gravity 1.053
12.25# Brit. Pale ale malt
.5# Wheat malt
.5# Crystal 40L
6 oz Crystal 120
1.4 oz Challenger 8% 60 min. 44.3 IBU
1.5 oz. Fuggles 5% 10 min. 5.8 IBU
1.5 oz Kent Goldings 5% 0 min.
Mash at 152 for 70 min
Notty yeast or Wyeast 1028
American IPA uses American base grain, 2 row - should have a high hop bitterness and use American hops - more citrusy, piney, fruity, and resiny. Malt flavors and aromas should be less than English IPA's. Wyeast 1056 or Safale US-05 and Mash at 149 for 90 min.
12.75# American 2-row
.75# Munich Malt
1# Crystal 15L
.25# Crystal 40L
Horizon 13% 60 min 49.7 IBU
Centennial 9% 10 min 6.9 IBU
Simcoe 12% 5 min 7.6 IBU
Amarillo 9% 0 min
Mash 149 for 90 min
Then there are Imperial IPAs with insane amounts of hops (well ) they have less crystal character than the American and English IPA's and they need to finish at a gravity between 1.012 - 1.015 ... Imperials have sugar added to them in order to get the gravity up... not only do they use a lot of hops but it is important to use the correct kinds/combinations of hops.
Color: 6 SRM
Boil 90 min, preboil gravity 7.74 gal. preboil gravity 1.062
15.25# American 2-row
.5# Wheat malt
1.5# Corn Sugar
.5# Crystal 40L
2oz Warrior 15% 90 min
2 oz Chinook 13% 90 min
1 oz Simcoe 12% 45 min
1 oz Columbus 14% 30 min
2.25oz Centennial 9% 0 min
1 oz Simcoe 12% 0 min
Use Wyeast 1056 or Safale US-05 make a 2L starter if you use 1056
Mash @ 150 for 90 min